This invention relates to a method for monitoring the thickness of aluminum oxide film formed on a moving aluminum foil which foil is intended for use in an electrolytic capacitor. More particularly, the invention is for continuously monitoring the potential electrical capacity of the formed foil as it exits an oxide formation machine. The term "formed foil" and variations thereof are used herein to mean anodizing, as is common practice in the capacitor art.
A formed foil is conventionally made by drawing a bare-aluminum foil through a liquid electrolyte and applying a voltage between the aluminum and the electrolyte as is described more fully by Gilbert et al in U.S. Pat. No. 3,962,048 issued June 8, 1976 and assigned to the same assignee as is the present invention.
The aluminum foil to be formed has normally been etched so as to increase the effective surface area per square area of the foil and therefore to increase the electrical capacity that can be obtained at each square of aluminum for a given thickness of aluminum oxide film. That oxide is to become the dielectric of an electrolytic capacitor. It should also be noted that in general, the electrical capacity is inversely related to the thickness of the oxide film.
It is well known that producing a uniform etch and a uniformly thick oxide film lead to tighter tolerance and/or lower cost capacitors. This is further elaborated by Arora et al in U.S. Pat. No. 4,279,714 issued July 21, 1981 and assigned to the above-said same assignee.
At the present time it is customary to periodically cut out a sample piece of the formed aluminum film, to submerse the sample piece in a beaker of electrolyte, to apply an AC voltage between the aluminum and the electrolyte, and to determine the ratio of reactive AC current to the AC voltage, from which the electrical capacity of the sample can be determined. Besides being time consuming, this sampling method often provides less than enough information to enable the determination of just the right quantity of the formed foil that should be incorporated in a capacitor of a given capacity, owing to variations in oxide film thickness and etch ratio along the length of the foil. It is therefore an object of this invention to provide a continuous method for monitoring the potential capacity and thus a measure of the oxide thickness of a formed aluminum foil.
It is a further object of this invention to provide such a continuous monitoring method that will be effective at the point where the formed foil exits a continuous forming machine.